This invention relates to the field of seismic exploration for subsurface mineral deposits and more particularly relates to such exploration as related to the location of subsurface stratigraphic traps containing petroleum.
Various techniques have been proposed in the prior art for locating subsurface stratigraphic traps. Stratigraphic traps have become of particular interest in recent times in petroleum prospecting, largely due to the advent of the so-called floating point amplifier seismic recording apparatus. The floating point amplifier enables a very accurate recording of the amplitude of acoustic energy reflected from subsurface acoustic impedance discontinuities at seismic detectors on the surface. Such acoustic impedance discontinuities occur at the boundaries of subsurface layering which have different mechanical properties for the reflection and transmission of acoustic energy. Changes in density, porosity, fluid or gas content or compaction can all lead to various differing travel times for acoustic energy in a subsurface formation.
In the prior art techniques for seismic exploration, stratigraphic traps which can comprise ancient river channels, sand bars, reefs, or the like, have been extremely difficult to locate because the reflection amplitudes and velocities of acoustic energy waves transmitted through these stratigraphic traps may be only slightly different from the surrounding rock layers. These traps may not exhibit characteristic anticlinal or dome structure generally associated with petroleum reservoirs. Such traps can contain enormous quantities, however, of high-grade crude petroleum deposits. The location of such traps is therefore of very high commercial interest to oil prospectors.
Little success has been realized in the prior art due to the fact that the acoustic energy properties of stratigraphic traps can be only slightly different from the properties of surrounding rocks. This, coupled with the almost non-existent structural characteristics of such traps, has made the task of recognizing their existence and determining their location in the subsurface by conventional seismic techniques very difficult. Thus, various attempts to make absolute measurements of the true velocity or true interval travel time, reflected energy amplitude, and/or frequency changes of seismic or acoustic energy associated with the presence of stratigraphic traps have often not been accurate enough to recognize and map stratigraphic traps. The method of the present invention, on the other hand, proposes a technique for locating stratigraphic traps wherein it is not necessary to achieve a high degree of absolute accuracy in the reflected seismic amplitude, velocity, or time measurement. The method of the present invention is based on recognizing the differential change of travel time of acoustic energy propagated through the stratigraphic trap from that propogated through the rock media surrounding the stratigraphic trap. The technique of the present invention, therefore, may be characterized as a differential method as opposed to prior art attempts at absolute measurements of the acoustic properties of subsurface earth formations.
Changes in the acoustic travel time for energy passing through a stratigraphic trap of commercial value and that passing through the surrounding rocks can be as short as 10 milliseconds. To make absolute measurements of time intervals this short on seismic records or record sections, or to make absolute amplitude measurements on reflected energy from the trap and the surrounding rock layers using prior art techniques would be exceedingly difficult with all but perfect seismic data. By observing the differences in apparent average velocity between closely spaced (relative to the lateral dimensions of the trap sought) surface locations in the manner of the present invention, however, changes in travel time on the order of ten milliseconds may be readily recognized using typical quality seismic data. This renders the present technique suitable for the location of stratigraphic traps. The present technique for locating stratigraphic traps has resulted from a study of the problems associated with the so-called velocity anomalies which have heretofore in the art been recognized only as a problem to be overcome in processing seismic data in order to clarify it for further interpretation by geophysicists and geologists in the search for subterranean petroleum deposits.